Improve the UI of Press & hold power button settings

The following changes has been made:
- The on/off toggle for Assistant now becomes two radio selectors ("Power Menu" and "Digital Assistant"). This is to make things clearer that Press & hold power button can be configured to power menu.
- The corresponding power menu animation will be shown when "Power Menu" is selected.
- The Assistant specific sensitivity settings and the foot notes are hidden when "Power Menu" is selected.
- Some minor wording updates according to the requests from Assistant / Settings UXW.

Bug: 229722937
Test: robotest, manual
Change-Id: Ib356fba861ad8c4a2626a0e0bd8cf3e4d90ce9a6